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Help please - voltage drop

Good day
I wired a db for a friend, we used 4mm2 twin and earth for 70m distance protected by a 35amp cb. I allready know the cable is to small for the voltage drop over the distance.But with no load i measure 212volts and 11 volts on the neutral. The elevated voltage on the neutral is what is bothering me. and also as soon as i put the plugs or aircon breaker on the voltage drops to less than a 100volt but there is no load so there should not be a vd????
Any help would be appreciated.
Johan

You say you 'wired a DB' using 4mm cable, is this actually a subDB running supplied from his main DB?
Did you export the earth or have you installed a spike at the remote location?
The 11 volts you have on the neutral, were you using a digital or analogue voltmeter and was this with reference to earth?

as soon as i put the plugs or aircon breaker on the voltage drops to less than a 100volt but there is no load so there should not be a vd????

I don't understand this part at all I'm afraid, please explain.

Can you say what load you will have on his cable ie air-con's, lighting, hot water heaters, etc?

I think that you may be measuring what I call capacitive induced voltage in the long cable. Basically a capacitor is two conductors, with a dielectric between them, so air is a a dielectric, and so is PVC plastic which usually covers the conductors. The closer and longer the length of the conductors, the higher the capacitance value, and because it is AC, it is transferred through the PVC. That is that there is power on this very long cable although not being consumed, is still there, and you are measuring the conductors against the earth. Now when you plug your equipment in, especially electronic devices, there are surge protection devices inside the equipment, along with capacitors between earth, live and neutral, which acts as suppressors and absorb electrical spikes that may be induced in the supply due to inductive loads being switched. So now the internal suppression components act as impedance's albeit high, still act against the cable and reduce the voltage as you have experienced. The moment you connect a load to the cable, you will get the readings you are expecting.

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what is the voltage at the main d.b. it should not drop 18 volts at the end of the cable with no load. i reckon the cable could be damaged,probably the neutral wire. check the n-e voltage at the main db. and compare to the end of the cable.